Packaging film

ABSTRACT

The present invention relates to packaging film for use in packaging of food products which may be heated whilst still packaged. In particular, the invention relates to packaging which may be hermetically sealed and, during heating, is able to allow a build up of pressure within the packaging, without the packaging exploding or bursting.

The present invention relates to packaging film for use in packaging offood products which may be heated whilst still packaged. In particular,the invention relates to packaging which may be hermetically sealed and,during heating, is able to allow a build up of pressure within thepackaging, without the packaging exploding or bursting.

With the ever-increasing demand for convenience foods, such as so-calledready-meals which merely require heating before consumption, there is arequirement for an economic method of rapid cooking. Consumers demandproducts which require as short a preparation time as possible, whilstmaintaining the quality of the heated food.

Cooking under pressure is a well known method of speeding up the cookingprocess. Not only can pressure cooking reduce the cooking time by 50%,if not more, but it can also result in a cooked product which is moresucculent and tender, due to the retention of moisture in the packagefor at least part of the cooking process.

The present invention seeks to exploit this known style of cooking inorder to improve the speed of food preparation and the quality of theheated food. However, in order to do this, it is necessary to not onlyprovide sealed packaging within which the pressure can build up, but toalso ensure that the pressure inside the package does not exceed thethreshold above which the packaging will explode or burst.

Various attempts have been made in the past to provide packaging which“self vents” above a certain pressure and/or temperature threshold.

U.S. Pat. No. 4,141,487 discloses packaging with a pleated cover film.An aperture is formed along one of the creases in the cover film, whichacts as a vent and releases the pressure which builds up within thepackaging during heating. Before heating, the aperture is hidden by thepleating and a thin layer of an adhesive sealant material is applied tohold the cover film in the pleated configuration and to keep the ventsealed within the pleat. The sealant is preferably one with a meltingtemperature which means that the pleat will unfold during cooking,thereby opening the vent and allowing vapour to escape as pressurebuilds up inside the packaging.

U.S. Pat. No. 4,404,241 disclose a packaging material having amulti-layer substrate with apertures. The apertures are sealed by acontinuous layer of an extrudable hot melt material which is bonded tothe substrate. When the packaging is heated, the hot melt materialbecomes viscous and permeable to vapour, thereby allowing venting.

U.S. Pat. No. 5,989,608 describes a complex valve structure in the coverfilm, to allow venting. The valve comprises a substrate film which ispositioned over an aperture in the cover film and is attached to thecover film by an adhesive layer. A separator film is positioned betweenthe adhesive layer and the aperture, thereby separating the adhesivelayer from the food contained within the packaging. Once the temperatureexceeds a given threshold, the adhesive allows the substrate film tobecome detached from the cover film, allowing venting through theaperture. Whilst such valves can be effective, they are also difficultto manufacture and can significantly increase the cost of the packaging.

The present invention seeks to provide packaging which self-vents once agiven temperature and pressure within the packaging has been reached.The packaging should also be capable of being hermetically sealed tomaintain the internal package atmosphere and extend the shelf life ofthe packaged product. Finally, it is also an aim of the presentinvention to provide packaging which is cheap and easy to manufacture.

The most common type of packaging used for ready-meals is a container,such as a tray, within which the food sits, the container being sealedusing a film which is welded to the container, usually along a flangesurrounding the opening of the container. That said, the presentinvention may, of course, be applied to other types of packaging, forexample bags.

In accordance with a first aspect of the invention, a packaging film isprovided comprising a carrier layer, a heat seal layer and a perforatedseal layer. When the packaging film is sealed, generally using theapplication of heat and pressure, the layers of thereof become welded toone another and to the rest of the packaging article. Sort specifically,the heat seal layer of the packaging forms an airtight seal between thecarrier layer and the perforated seal layer in the area where thepackaging film is sealed. Thus, the package is hermetically sealed.

In one embodiment, the heat seal layer is applied to the carrier layer.The heat seal layer is then intermittently attached to a perforated seallayer using an adhesive. This can be achieved, for example, by usingstrips of adhesive.

In an alternative embodiment, the heat seal layer is applied to theperforated seal layer. Optionally, the heat seal layer is thenintermittently attached to the carrier layer using an adhesive.

In yet another embodiment, the heat seal layer is applied to both thecarrier layer and the perforated seal layer. The two heat seal layersmay then be intermittently attached to one another using an adhesive.

In a further preferred embodiment, no intermittent adhesive is used.Rather, the carrier layer and the perforated seal layer are adhered toone another by the heat seal layer upon sealing the packaging film, withthe application of heat and pressure. This embodiment works when theheat seal layer is applied to either the carrier layer, the perforatedseal layer, or both.

When a packaging article sealed with a packaging film according to thefirst aspect of the present invention is heated, pressure will start tobuild up inside the container, allowing pressure cooking. When a certaintemperature and pressure has been reached within the container, the heatseal layer positioned between the carrier layer and the perforated seallayer becomes gas permeable and may even give way completely. At thispoint, the pressure from inside the package will be released to theexterior of the container through channels formed by the perforations inthe perforated seal layer and the areas between the carrier layer andthe perforated seal layer which were joined by the heat seal layer,which is now gas permeable or has given way. Once such venting hasoccurred, a balanced state is maintained, as any excess pressurecontinues to vent out of the container where the heat seal lacquer havegiven way.

The heat seal material used is chosen to provide venting at a desiredtemperature and pressure. Preferably, venting occurs at a temperaturebelow 150° C., and more preferably below 100° C. It may also beadvantageous to use heat seal material which becomes viscous and gaspermeable at elevated temperatures, but which is not permeable toliquids. This will allow release of the pressure, whilst keepingmoisture sealed within the packaging, thereby keeping the heated foodsucculent. It will also prevent undesirable spurting of hot liquid outof the container once venting has started.

Heat seal materials which may be used in the packaging film of thepresent invention include: polyester film with a heat seal layerextruded or coated onto it with or without a primer; cast or blownpolypropylene film; a laminate of polyester or nylon film to cast orblown polypropylene film; polyethylene film; and laminate of polyesteror nylon film to cast or blown polythene film or other heat-sealablepolymers.

The carrier layer can be made from a variety of known materials, such aspaper, nylon, cellophane or polyester. The material must be thermallystable, and should also be impermeable to gas and liquid. Thisimpermeability may be achieved by applying an impermeable coating to thecarrier layer material, if necessary. Alternatively, the layer of heatseal material applied to the carrier layer may act as a barrier to gasand liquid prior to heating. The preferred material for the carrierlayer is polyester although alternative materials which are impermeableto gas and suitable for contact with food materials could also be used.

Appropriate materials for the container should also be selected, with aview to the proposed mode of heating. For example, a polypropylenecontainer would be suitable for microwave heating, but not for heatingin a conventional oven. CPET containers are commonly used to packageready-meals, as they can be heated in both microwave and conventionalovens. Preferably, the container is made from a material which retainsits structural integrity at high temperatures and does not become softor pliable which will make the container difficult to carry or make itmore difficult to remove the covering film.

The perforated seal layer is preferably selected for optimal adhesion tothe material from which the container or the rest of the packaging isformed. In one embodiment, the perforated seal layer material ispolyester. Such a perforated polyester seal layer is heat-sealable to aCPET tray. However, a polyester perforated seal layer will not sealsatisfactorily to certain materials, such as polypropylene. Thus, if thecontainer were composed of, for example, polypropylene, then theperforated seal layer could either be formed from polypropylene alone,or it could be a laminate of materials, including a layer ofpolypropylene adjacent to the container.

In another embodiment of the invention, a packaging film is providedcomprising a carrier layer which is intermittently adhered to aperforated seal layer. The film does not include a heat seal layer. Apackaging article incorporating such a packaging film will not behermetically sealed, i.e., the contents of the packaging will be open tothe atmosphere to a certain extent. Thus, this packaging film isbreathable and is useful for the packing produce such as fruits andvegetables. However, the packaging will still allow a degree of pressurebuild-up upon heating, thereby allowing pressure cooking of thepackaging contents to a certain extent.

In a further preferred embodiment of the invention, the packaging filmaccording to the invention includes a so-called susceptor film. Asusceptor film is a lightly metalised film which acts as a resistor tomicrowaves. As a susceptor film is exposed to microwave radiation, itbecomes hot and acts as a cooking instrument, providing a source ofintense heat. A normal metalised film with an optical density of 2 to2.5 units acts as a conductor and leads to arcing in a microwave oven.In contrast, a susceptor film that is metalised with an optical densityof 0.2 to 0.4 acts instead as a resistor to microwaves and thereforeheats up to high temperatures, in excess of 200° C.

The incorporation of a susceptor film in the packaging film combines theeffect of “super-heat” and pressure cooking. This can reduce the cookingtime to around 25% of the normal time required to cook the same product.In addition, the consumer requires no additional cooking utensils or thelike.

A susceptor film is usually made by metalising a polyester film. Becausethe susceptor film heats up to extreme temperatures when microwaved andthe polyester film is thermoplastic, the susceptor film will shrivel upand melt during cooking. Therefore, in order for the susceptor film towork, it should be laminated to a non-thermoplastic material such aspaper or cellophane, which stabilises it.

The susceptor film can be incorporated into any of the embodiments ofthe first aspect of the present application and it is preferably addedto the perforated seal layer or the carrier layer. Some of theembodiments including a susceptor film are illustrated in the figuresand are described in detail below.

According to a second aspect of the present invention, a packagingarticle is provided, comprising a packaging film according to the firstaspect pf the present invention.

In one embodiment, the packaging article comprises a container, such asa tray, the opening of which is sealed using a packaging film accordingto the first aspect of the invention.

In an alternative embodiment, the packaging article is a bag, at leastpart of which is formed from a packaging film according to the firstaspect of the present invention.

The bags could be made and then filled, or they could be made and filledsimultaneously using appropriate machinery.

Embodiments of the invention are now described by way of example only,with reference to the accompanying drawings, in which:

FIGS. 1 to 7 each show a schematic cross-sectional view of a packagingfilm in accordance with an embodiment of the first aspect of the presentinvention; and

FIG. 8 shows a schematic cross-sectional view of a packaging articleaccording to the second aspect of the invention.

FIG. 1 shows a packaging film wherein the carrier layer 10 is coatedwith a heat seal layer 16. The heat seal layer is strip laminated to aperforated seal layer 14 using strips of adhesive 12. Channels 18 areformed between the perforations in the perforated seal layer 14 and theareas where there is no intermittent adhesive 12 between the carrierlayer 10 and the perforated seal layer 14. When the packaging film issealed to a container, the carrier layer with the heat seal layerbecomes attached to the perforated seal layer and the container, and thechannels 18 are sealed. When the packaging is heated, the heat seallayer releases, opening the channels and allowing venting.

FIG. 2 shows an embodiment wherein the heat seal layer of the packagingfilm is applied to the perforated seal layer, and it is adhered to thecarrier layer using strips of adhesive.

In the embodiment shown in FIG. 3, the heat seal layer is applied toboth the carrier layer and the perforated seal layer. Strips of adhesiveare positioned between the two heat seal layers.

FIG. 4 shows an alternative embodiment, wherein there is no intermittentadhesive present. The layers of the packaging film will be adhered toone another upon welding.

FIG. 5 shows an alternative embodiment of the present invention, whereinthe perforated seal layer comprises a laminate of two differentmaterials. Thus, for example, the layer 20 is polyester, whilst layer 22is polypropylene. Such a film may be sealed to a propylene container.

FIG. 6 illustrates a further embodiment of the present invention,wherein the packaging film includes a susceptor film added to theperforated seal layer, forming a susceptor laminate. The susceptorlaminate comprises a sheet of cellophane or paper 32, a layer ofadhesive 26, susceptor metalisation 24, a sheet of polyester to whichsusceptor metalisation is applied 28, and a perforated seal layer 14.

FIG. 7 shows a packaging film whereby the susceptor film is added to thecarrier layer, forming a susceptor laminate. The susceptor laminateincludes a layer comprising cellophane or paper 32, a layer of adhesive26, susceptor metalisation 24, a sheet of polyester to which susceptormetalisation is applied 28, and a heat seal layer 16.

FIG. 8 shows a packaging film according to the first aspect of theinvention, sealed to a container 30, forming a packaging articleaccording to the second aspect of the invention.

Permutations and combinations of various described embodiments of theinvention can be used, whilst retaining the desired basic properties ofthe packaging film and/or packaging article described herein.

1. A packaging film comprising: a carrier layer which is impermeable togases and fluids and a perforated seal layer which may be heat sealed toa packaging material to which the packaging film is to be attached,wherein the carrier layer and the perforated seal layers are adhered toone another directly upon sealing the packaging film to a packagingmaterial opening with an application of heat and pressure wherein theheat seal releases when heated to a predetermined temperature to allowthe packaging material to vent during heating.
 2. A packaging filmcomprising: a carrier layer which is impermeable to gases and fluids,having two sides; a perforated seal layer having two sides; and a heatseal layer which is impermeable to gases and fluids, applied to one ofthe two sides of either the carrier layer or the perforated seal layer;wherein the carrier layer and the perforated seal layer are adhered toone another by the heat seal layer upon sealing the packaging film to apackaging material and said packaging film hermetically seals saidpackaging material, opening with an application of heat and pressure;wherein the heat seal layer releases when heated to a predeterminedtemperature to allow the packaging material to vent during heating.
 3. Apackaging film comprising: a carrier layer which is impermeable to gasesand fluids; a heat seal layer which is impermeable to gases and fluids,applied to the carrier layer; and a perforated seal layer, whereby theheat seal layer is positioned between the carrier layer and theperforated layer; wherein: the perforated seal layer comprises aplurality of perforations forming channels which are closed when thepackaging film is sealed to a packaging material and said packaging filmhermetically seals said packaging material; and the heat seal layerreleases when heated to a predetermined temperature allowing saidchannels to open to allow venting during heating.
 4. A packaging filmaccording to claim 3, wherein the heat seal layer is intermittentlyattached to the perforated seal layer by an adhesive.
 5. A packagingfilm according to claim 3, wherein the heat seal layer and theperforated seal layer are attached by strips of adhesive.
 6. A packagingfilm according to claim 3, wherein the carrier layer is polyester,nylon, cellophane, or coated paper.
 7. A packaging film according toclaim 3, wherein the perforated seal layer comprises polyester.
 8. Apackaging film according to claim 3, wherein the perforated seal layeris a laminate of two or more materials.
 9. A packaging film according toclaim 8, wherein one of the materials of the laminate is polypropylene.10. A packaging film according to claim 3, wherein the film furtherincludes a susceptor film comprising a lightly metallized film.
 11. Apackaging film according to claim 10, wherein the susceptor film islaminated to a non-thermoplastic material.
 12. A packaging filmaccording to claim 10, wherein the susceptor film has an optical densityof 0.2 to 0.5.
 13. A packaging film according to claim 10, wherein thesusceptor film is incorporated into the perforated seal layer.
 14. Apackaging film according to claim 13, wherein the susceptor film isperforated.
 15. A packaging film according to claim 10, wherein thesusceptor film is incorporated into the carrier layer.
 16. A packagingfilm comprising: a perforated seal layer comprising a plurality ofchannels; a heat seal layer which is impermeable to gases and fluids,applied to the perforated layer; and a carrier layer which isimpermeable to gases and fluids, attached to the heat seal layeropposite the perforated seal layer; wherein the channels are sealed whenthe packaging film is sealed to a packaging material and said packagingfilm hermetically seals said packaging material; and the heat seal layerreleases when heated to a predetermined temperature to allow thechannels to become unsealed during heating.
 17. A packaging filmaccording to claim 16, wherein the heat seal layer is intermittentlyattached to the carrier layer by an adhesive.
 18. A packaging filmaccording to claim 16, wherein the heat seal layer and the carrier layerare attached by strips of adhesive.
 19. A packaging film according toclaim 16, wherein the carrier layer is polyester, nylon, cellophane, orcoated paper.
 20. A packaging film according to claim 16, wherein theperforated seal layer comprises polyester.
 21. A packaging filmaccording to claim 16, wherein the perforated seal layer is a laminateof two or more materials.
 22. A packaging film according to claim 21,wherein one of the materials of the laminate is polypropylene.
 23. Apackaging film according to claim 16, wherein the film further includesa susceptor film comprising a lightly metalized film.
 24. A packagingfilm according to claim 23, wherein the susceptor film is laminated to anon-thermoplastic material.
 25. A packaging film according to claim 23,wherein the susceptor film has an optical density of 0.2 to 0.5.
 26. Apackaging film according to claim 23, wherein the susceptor film isincorporated into the perforated seal layer.
 27. A packaging filmaccording to claim 26, wherein the susceptor film is perforated.
 28. Apackaging film according to claim 23, wherein the susceptor film isincorporated into the carrier layer.
 29. A packaging film comprising: acarrier layer which is impermeable to gases and fluids, having twosides; a first heat seal layer which is impermeable to gases and fluids,applied to one of the two sides of the carrier layer; a perforated seallayer having two sides; and a second heat seal layer applied to one ofthe two sides of the perforated seal layer, wherein the carrier layerand the perforated seal layer are adhered to one another by the heatseal layers upon sealing the packaging film to an opening of a packagingmaterial with an application of heat and pressure, and said packagingfilm hermetically seals said packaging material, wherein the heat seallayers release when heated to a predetermined temperature to allow thepackaging material to vent during heating.
 30. A packaging filmcomprising: a carrier layer which is impermeable to gases and fluids,having two sides; a first heat seal layer which is impermeable to gasesand fluids, applied to one of the two sides of the carrier layer; aperforated seal layer having two sides comprising a plurality ofperforations forming channels; and a second heat seal layer applied toone of the two sides of the perforated seal layer, wherein the two heatseal layers are intermittently attached by an adhesive; wherein thechannels are closed when the packaging film is sealed to a packagingmaterial, and the heat seal layers release when heated to apredetermined temperature to allow the channels to open during heating.31. A packaging film according to claim 30, wherein the two heat seallayers are attached by strips of adhesive.
 32. A packaging filmaccording to claim 30, wherein the carrier layer is polyester, nylon,cellophane, or coated paper.
 33. A packaging film according to claim 30,wherein the perforated seal layer comprises polyester.
 34. A packagingfilm according to claim 30, wherein the perforated seal layer is alaminate of two or more materials.
 35. A packaging film according toclaim 34, wherein one of the materials of the laminate is polypropylene.36. A packaging film according to claim 30, wherein the film furtherincludes a susceptor film comprising a lightly metalized film.
 37. Apackaging film according to claim 36, wherein the susceptor film islaminated to a non-thermoplastic material.
 38. A packaging filmaccording to claim 36, wherein the susceptor film has an optical densityof 0.2 to 0.5.
 39. A packaging film according to claim 36, wherein thesusceptor film is incorporated into the perforated seal layer.
 40. Apackaging film according to claim 39, wherein the susceptor film isperforated.
 41. A packaging film according to claim 36, wherein thesusceptor film is incorporated into the carrier layer.
 42. A packagingarticle comprising a packaging film as claimed in any one of claims 1,16, 30, 29 and
 2. 43. A packaging article according to claim 42, whereinthe article further comprises a container having at least one openingwhich is closed by the packaging film wherein the perforated seal layerof the packaging film is in contact with the container.
 44. A packagingarticle according to claim 43, wherein the container has a flange aroundthe at least one opening and the packaging film is sealed to the flangeby an application of heat and pressure.
 45. A packaging articleaccording to claim 43, wherein the container is composed ofpolypropylene and the perforated seal layer comprises a polypropylenelayer adjacent to the container.
 46. A packaging article according toclaim 43, wherein the perforated seal layer comprises polyester and thecontainer is composed of crystalline poly (ethylene terephthalate) withor without an amorphous poly (ethylene terephthalate) layer.
 47. Apackaging article according to claim 42, wherein the article is a bag,at least a portion of which is formed from the packaging film.
 48. Apackaging article according to claim 42, wherein the perforated seallayer and the heat seal layer seal the packaging article and preventegress of gas or liquid.
 49. A packaging article according to claim 42,for heating food products contained therein in a microwave or aconventional oven.